CUE can handle many types of cycle, resolving and inferring values that depend on each other.

Because all values are final in CUE, a field with a concrete value (e.g. 200) can only be valid if it is that value. If CUE sees this concrete value being unified with some other expression then the evaluation of that expression is postponed, which often allows cycles to be broken.

This can prove useful for template authors who know that a user will provide one out of several fields, but they may not know which one.

x: 200

// This cycle can be resolved because of the
// concrete value specified above.
x: y + 100
y: x - 100

// This cycle cannot be resolved, so "cue eval"
// report the field's values as their given
// expressions.
a: b + 100
b: a - 100
$ cue eval file.cue
x: 200
y: 100
a: b + 100
b: a - 100

This example uses cue eval because the cycle between the a and b fields can’t be resolved. Using cue export would fail because the fields’ values aren’t concrete.